Selective paging system



Dec. 10, 1963 H. w. BODE ETAL 3,114,142

SELECTIVE PAGING SYSTEM Original Filed Feb. 11, 1955 5 Sheets-Sheet 1 TSUBSCR/BER c RELE/l/ER Q DECODER RECEIVER C) /DECODER SUBSCRIBER A 24 RECEIVER 25V /DECODER 27- F IG. 3

OUTPUT 34 34 34 I I I OFDELAY A B C 0 E F A B c 0' E F CHANNEL I I I l 1 I /v0.o 0 0 0 5 as NO./ o 0 0 N0.5 0 0 0 TIME 1 "FIR/N6 swr" HJ. Mc SK/M/N BY A TTOR/VEV Dec. 10, 1963 H. w. BODE ETAL SELECTIVE PAGING SYSTEM 5 Sheets-Sheet 2 Original Filed Feb. 11, 1955 HW 8005 lNl/ENTORS. HJ MC SK/M/N ATTORNEY 10, 1963 H. w. BODE ETAL 3,

SELECTIVE PAGING SYSTEM Original Filed Feb. 11, 1955 5 Sheets-Sheet 3 FIG. 4

A 0 HOLD ava n'OLD v AMI? I73 1;. ava

D- -O 0- i HOLD (/38) I l IS/ 12 HOLD (/54) I L.

START OF OUTPUT SIG/VAL HM! BODE ATZIOR EZ Dec. 10, 1963 H. w. BODE ETAL 3,114,142

SELECTIVE PAGING SYSTEM Original Filed Feb. 11, 1955 5 Sheets-Sheet 4 FIG. 6

H w 8005 M H.J. Mc SK/M/N M Man ATTORNEY Dec. 10, 1963 H. w. BODE ETAL SELECTIVE PAGING SYSTEM Original Filed Feb. 11, 1955 5 SheetsSheet 5 FIG. 7

FIG. 8

o D TIME TIME H. w 8005 H. J. M: SKlM/N y ATTO United States Patent Ser- No. 487,686. 3, 1960, Ser. No.

2 Claims.

This invention relates to pulse code recognition circuits for use in selective paging systems, and more particularly to circuits of this type which are adapted to be conveniently carried on the person of the individual to be paged.

This application is a division of copending application Serial No. 487,686, filed February 11, 1955, now Patent 2,955,279.

It is important for many people such as physicians, executives and the like to be continuously in contact with their oifices or their homes. However, the nature of their work may involve frequent moving from one location to another and/ or remaining out of contact with conventional prior art communication facilities for substantial periods of time. While a mobile radio'telephone would in some instances be ideal, such equipment is normally too heavy to be conveniently carried on the person and, in many instances, too expensive to be extirely satisfactory. A radio paging system is an alternative method for contacting persons having the requirements noted above. The subscriber in a radio paging system of the present invention would carry a radio receiver, including a pulse code recognition circuit, which would give an audible signal when an associated radio transmitter broadcasts a predetermined coded signal. The subscriber would then telephone or return to his ofiice to check on the reason for the call.

A paging receiver of the invention must recognize a predetermined coded signal and respond to it and to no other signal. The coded signal is normally in the form of a group or several groups of electrical pulses. Many circuits have been proposed heretofore which recognize or identify coded pulse signals. In general, however, such circuits are too elaborate and bulky to be conveniently carried on the person, and are not adapted for various other mobile uses for the same reasons.

Accordingly, the principal object of the present invention is to simplify, and to reduce the weight and bulk of, radio receivers including pulse code recognition circuits.

Another object of the invention is to reduce the power requirements of pulse code recognition circuits.

In accordance with one aspect of the present invention, pulse code recognition circuits are simplified by the use of a common timing circuit for converting coded pulse groups into simultaneous coded signal indications, in combination with separate enabling circuits energized respectively by successive code groups applied to said timing circuit. Another aspect of the invention involves the use of pulse code groups having initial and terminal pulses forming a time frame of reference. The use of a terminal timing pulse permits the elimination of additional timing equipment which would otherwise be required at the receiver.

A feature of a number of circuits embodying the principles of the invention is the use of transistor circuitry in which the transistors are normally cut oil, and draw substantial amounts of current only when the circuits are actuated. This greatly reduces the battery drain during standby intervals and facilitates the reduction in weight of the portable equipment.

Other objects and certain additional features and ad vantages will become apparent during the course of the Patented Dec. 10., l9fi3 following detailed description, from the accompanying drawings, and from the appended claims.

In the drawings:

FIG. 1 is a schematic diagram of a paging system;

FIG. 2 is a circuit diagram of a pulse code recognition circuit in accordance with the invention;

FIG. 3 is a diagram which indicates the electrical conditions at various points in the circuit of FIG. 2;

FIG. 4 is a block diagram of another form of pulse code recognition circuit in accordance with the invention;

FIG. 5 is a pulse diagram indicating the relationship of pulses applied to the circuit of FIG. 4;

FIG. 6 is a detailed circuit diagram of the circuit of FIG. 4;

FIG. 7 is a holding circuit which is used in the circuit of FIG. 6 and which may also be employed in the circuit of FIG. 2;

FIGS. 8 and 9 are plots of the characteristics of the circuit of FIG. 7; and

FIG. 10 is a triggered oscillator which may be used as the signaling device of FIGS. 2 or 6.

Referring more particularly to the drawings, FIG. 1 illustrates a paging system in accordance with the present invention. In FIG. 1, the paging system transmitter 21 transmits pulse code signals from its antenna 22. These signals are transmitted simultaneously to a large number of paging receiver units, each being associated with a particular subscriber, as illustrated, for example, by subscriber A, subscriber B and subscriber C in FIG. 1. Each subscriber has a unit which includes a receiver 24 and a decoder unit 25. These units are very compact and lightweight and may be carried in a coat pocket or in a briefcase. The receiver includes an antenna 26 which may be a singlestrand of flexible wire concealed in the clothing of the subscriber. The decoder is equipped with a compact loudspeaker 27 which fits into the side of the case enclosing the decoder.

The pulses may be transmitted from the paging system transmitter 21 to the receiving units of subscribers A, B and C at any suitable radio frequency, and these signals may be modulatedin any desired manner. The output from the receiver 24 to the decoder 25, however, may be in the form of intermediate frequency pulses. By way of specific example, these intermediate frequency pulses may occur at two microsecond intervals, and may have a carrier frequency of fifteen megacycles per sec end. The fifteen megacycle frequency was selected for optimum performance of the fused silica delay lines which were employed.

The intermediate frequency pulses from receiver 24 are applied at input terminal 31 of the decoder unit shown in FIG. 2., By way of example, the applied groups of pulses may have the form shown at 34 and 35 in FIG. 3. The groups of pulses are applied to the delay channels d through d inclusive, in FIG. 2. These delay lines may be, by way of example, rods of fused silica having barium titanate transducers at each end. These electroacoustic delay lines d through d delay accurately the applied signals by time intervals corresponding to the length of the rods.

The barium titanate transducers 41 through 45, inclusive, are secured to the input ends of the silica rods, and transducers 41' through 45 are attached to the output ends of the rods. They serve to convert electrical pulses to mechanical waves, and vice versa. The input barium titanate' transducers 41 through 45 are connected j in series in order to present a higher input electrical impedance to the intermediate frequency pulses applied to terminal 31. The coil 46 is provided to tune the input at terminal 31. The output impedances of the transducers 41 through 45, inclusive, are also tuned through the use of the coils 51 through 55, respectively. Intera mediate frequency signals from delay lines d through d are then applied to the rectifying elements 61 through 65', inclusive. The rectified output from each of the diodes 61 through 65 applies a positive current pulse to the base of the corresponding N-PN transistor 71 through '75. The current flowing in the output circuit of one of the transistors 71 through 75 produces a pulse on the primary winding 81A through 85A of the appropriate transformer 81 through 35.

Each of the transformers 81 to 85, inclusive, and 94 includes a single primary and a plurality of secondary windings. The primary windings are designated 81A to 85A inclusive, and 94A; and successive secondary windings are designated by the letters B and C. Thus, the first group of secondary windings 8113 to 85B inclusive and 94B are shown opposite their respective primary windings 81A to 35A, inclusive, and 94A. The second set of secondary windings 81C to 85C, and 94C, are shown below the main circuit diagram to simplify the circuit layout.

A zero delay channel d is also provided. This channel includes the resistance 91, the diode 92, the transistor 93 and the pulse transformer 4. The purpose of the re istance 91 is to compensate for the loss in the delay lines 11 through d and thus equalize signal levels at the pulse transformers '81 through 85 and 94.

The NPN transistors 71 through '75 and 93 require a positive input signal to produce an output pulse. The secondaries of the transformers 81 through 85 and 94 are connected to a circuit including the transistor 96. The transistor 96 is a PNP transistor, and therefore requires a negative input pulse in order to produce an outp t pulse. The base to emitter circuit of this transistor is, however, biased three volts positive by the battery 97. The transformer secondaries 8113 through 8513 and 94B are respectively connected to the load resistances 181 through 186, inclusive, in the input biasing loop of the transistor 9:6. An appropriately poled pulse across one of the resistances 1491 through 1% inserts a negative bias of 1.5 volts into the input biasing loop. Accordingly, three appropriately poled pulses from the pulse transformer secondaries 813 through 858 and 94B are required to provide net negative voltage in the loop, and to energize the transistor 96.

As shown in FIG. 3, the pulse code applied to the delay channels d tified by the letters A through F. The initial and final pulses A and F, respectively, form a part of each pulse group which is applied to the input terminal 31 of the decoder unit. The signal information is transmitted by the presence or absence of pulses in the time slots B, C, D and B. As indicated by the pulse group 34 in FIG. 3, pulse C is the only signal information pulse present in this group of time slots B through E.

In the circuit of FIG. 2, the pulse transformer secondary windings are connected to recognize the signal pattern identified as pattern 34 in FIG. 3. Thus, the secondnries of transformers 83, 85 and 94 are connected into the series loop in such a manner as to produce voltages which tend to cancel the bias of battery 97. The secondary windings of the transformers 81, 52 and 84, however, are connected into the series loop to provide voltages which increase the bias, which prevents operation of transistor 96. This dir'ierence in polarity is shown in FIG. 2 by the direct connections of the secondaries 83B, 85B and MB, as contrasted with the crossed connections of the secondaries 81B, 82B and 843 in their connections to the biasing circuit loop to the base of transistor 96. Thus the presence of an unwanted pulse in an otherwise correct group of code pulses produces a voltage which prevents the operation of the paging systei In the diagram of FIG. 3, the output of each delay channel is plotted versus time. A in the chart of FIG. 3 indicates the absence of a pulse at the appropriate through d is related to six time slots iden-- be observed that this combination of three signs only occurs during the instant labeled firing slot, and in no other time intervals. It may also be readily determined that no other combination of pulses will energize the input to transistor 96. 7

When the transistor 9a is energized, a pulse is applied to the transformer 111, including the primary 112 and the secondary 113. The stray capacitance 114 resonates vith the inductance of the transformer 111 and an oscillation is established. The next succeeding code group A through F designated 35 in FIG. 3 is applied to input terminal 31, whilethe oscillation associated with transformer 111 is still maintained. The transformers 81 through 85 and )4 each have two secondary windings. As mentioned above, a second group of these secondary windings is indicated at 81C through 85C and 94C in FIG..2. T e connections of these secondary windings to the resistances designated 118 in PEG. 2 correspond to the pulse pattern 35 of FIG. 3. pulse pattern 35 appears at the secondary windings 81C through 85C and 94C, the negative bias 119 associated with N-P-N transistor 1 .21 is overcome, and this transistor 121 is also energized.

The energization of transistor 121, together with the oscillations present in transformer 111, are sufiicient to energize transistor 123. This transistor in turn energizes the amplifying transistor 124, and a pulse is pro duced at the output speaker 12-5. The transformer 127 is employed to intercouple the transistor stages 123 and 124. The transformer coupling permits the use of a single battery 128 for supplying power to the collectors of transistors 96, 123 and 124. The loud-speaker unit 126 may be a small magnetic unit designed to be used in a sound powered telephone or other suitable signaling device. As indicated at 27 in FIG. 1, the speaker is mounted in the side of the decoder unit. The condenser 129 helps to match the input impedance of the speaker 126 to the output of the transistor 124. The transformer 81 through 85 and 94 may have a third set of secondary windings (not shown) connected thereto; and these may be used to provide a greater number of unique pulse combinations by the use of a third pulse code group similar to those shown at 34 and 35 in FIG. 3. V

FIG. 4 is a block diagram of another form of pulse decoding circuit. Instead of the binary coding system employed in the circuit of FIG. 2, the arrangement of FIG. 4 employs a pulse position modulation coding arrangement. For example, with the pulses being applied at input terminal 131, the delay lines r1 together with the zero delay line d provide'a timing circuit. This timing circuit is designed to recognize pulse patterns of the f rm shown in FIG. 5. For example, the spacing between pulses 133 and 134 of FIG. 5 corresponds to the delay introduced by the delay unit (i The diodes 171 through 174 are shown at the output of delay lines d through c1 respectively to indicate the transformation from intermediate frequency pulses applied at terminal 131 to direct current 7 pulses. V

The gate 136 is a coincidence gate or AND and yields an output if both input leads are energized. However, no output will be produced if only one of the two input leads of the AND unit receives a pulse. Accordingly, when the undelayed pulse 134 appears by way of the zero delay line d at the same instant that Accordingly, when the (111' (212' andunit,

the delayed pulse 133 appears at the output of delay unit d the coincidence gate 136 will be energized, and the holding circuit 138 will be actuated. Similarly, the pulse pattern 141, 142 will energize the AND unit 144; and the pulse pattern 146, 147 of FIG. 5 will energize the third AND unit 149. The energization of the holding circuit 138 is indicated in FIG. 5 by the waveform 151. The holding circuit 138 energizes one of the inputs to the AND unit 152 for an extended period which includes the brief moment when an output pulse is produced by AND unit 144. At this moment, the hold circuit 154 is energized. The time period of the hold circuit 154 is indicated at 156 in FIG. 5. The hold circuit 154 remains energized until after the arrival of an input pulse from AND circuit 149. The concurrent energizatic-n of the two input leads ofcoincidence gate 158 produces an output pulse to the amplifier 159, and energizes the signaling loudspeaker 161.

The detailed circuit diagram of FIG. 6 corresponds to the block diagram of FIG. 4. As in the case of the circuit of FIG. 2, the input pulses are applied in series to the delay lines (i (1 and c1 A resistance element 163 is again inserted in the zero delay line d to equalize the output level of this line to that of the other delay lines. Inductances 164, 165 and 166 are provided to tune the output capacitance of the delay lines du, dlg and 11 respectively. The diodes 171 through 174 and the transistors 175 through 178 also serve the same rectifying and amplifying functions as the corresponding elements in the circuit of FIG. 2. It may also be noted that the inductances 164, 165 and 166 provide a direct current path for the rectified output of the diodes 172, 173 and 174. The output from transistors 175 through 178 is developed across resistances 167 through 178, respectively. A pulse transformer having a primary 183 and three secondaries 181, 191 and 198 plays an important part in the present circuits. The primary winding 183 is connected to the output of the zero delay circuit d in the collector circuit of the transistor 175. The secondary windings 181, 191 and 198, however, are respectively connected in series with the output circuits of delay lines d d and d When pulses appear simultaneously at the output of transistors 175 and 176, the output developed across resistance 168, combined with the output of the secondary winding 181, is suificient to overcome the bias of battery 182., in the base circuit of transistor 185, and the transistor is energized. The value of the biasing battery 182 is such that a pulse at the output of only one of the transistors 175, 176 is insuflicient to tire the transistor 185. The energization of the transistor 185 energizes the hold circuit including the transistor 187, which will be described in detail in connection with FIGS. 7 through 9. Among other functions, the transformer 188 isolates successive transistor stages, and permits the use of a single battery 189. When the second group of pulses identified as pulses 141 and 142 in FIG. 5 appear at the output of the transistors 175 and 177, the transistor 193 will be energized. The concurrent energization of the hold circuit, including transistor 187 and transistor 193, energizes the second hold circuit which includes transistor 195. The transformer 197 is also employed for holding and isolation purposes. The third group of pulses designated 146, 157 in FIG. 5 energizes the primary 183 at the same instant that a pulse from delay line d13 energizes transistor 178. The combined voltage developed by the transformer secondary 198 and the load circuit 171) of the transistor 178 is sufiicient to trigger the transistor 281. The output from the transistor 281 is coupled by the isolating transformer 202 to the transistor amplifying stage 2115.

The hold circuit, including transistor 185 and transformer 1188, will now be described in somewhat greater detail by reference to FIG. 7. The negative pulse required to trigger the P-NP transistor 185 is indicated at 20 8. The output pulse from the transistor 185 creates a surge of current in the inductance associated with the transformer 188. The stray capacitance of the transformer and its associated wiring is indicated at 28 9. The oscillation which starts in the tuned circuit is indicated at 211 in FIG. 8. If the transistor 187 were disconnected from the circuit, the oscillation would continue as indicated at 12112 in FIG. 8. The plot of FIG. 8 represents the voltage at point 214, which is the input lead to the base of the transistor 187. When the voltage across the base to emitter circui of the tnansistor-187 is positive, this element is efiectively out of the circuit. When the voltage becomes negative, however, the base to emitter circuit assumes its low resistance condition, and conducts current readily. At this instant, essentially all of the energy is in the magnetic iield of the inductance, so that the ensuing decay is that of an inductance loaded with a nonlinear resistance. The current flowing out of the base of the transistor :187 is indicated by the plot 216 of FIG. 9. As the current flowing out of the transistor base decreases, the base to emitter resistance increases so that the power dissipated terminates the decay rather abruptly, as indicated by the portions 217 and 218 of the solid line plots ofFlGS. 8 and 9, respectively. 1

During the period of time while current flows in the base circuit of transistor 187, a negative pulse on the base of transistor 193 energizes its load circuit, including the impedance element 216. The impedance 2116 may represent another holding circuit or may represent the alert signal.

FIG. 10 represents a triggered oscillator. The triggered oscillator in FIG. 10' may be employed in place of the pulse output signal circuits of (FIGS. 2 and 6, respectively. When the circuits of FIGS. 2 and 6 are employed without the triggered oscillator of FIG. 10', the pulse code is repeated.at an audio rate, and thus yields a tone output at the speaker unit. To consenve transmitter channel time, however, it may be desirable to use an oscillator such as that showvn in F-IG. 10 which is triggered by a single correct pulse code. The tnansistor 221 of FIG. 10 may correspond to the transistor 124 of FIG. 2, or to transistor 205 of FIG. 6. The output of transistor 221 is coupled by means of the resistance 222 and the capacitance 223 to the transistor 226. The transistor 2126is the active element in a transformer coupled oscillation circuit which includes the transformer 9128, and the loudspeaker device 2129. Power is supplied to the tran- This energizes the speaker 297, which is located in the 1 collector circuit of the transistor 285. The condenser 296 helps to match the speaker 207 to the output circuit of the transistor 205.

sistors 20.11 and 226 by the batteries 231 and 232., respectively. A switch Q34 is provided to stop the output signal from the oscillator by grounding the base to emitter circuit of the transistor 226.

As mentioned above, one type of delay line which operated satisfactorily was made of thin rods of fused silica. Delay lines made of this material have accurate timing properties, very low acoustic loss, and do not change their delay properties substantially with temperature variations. While acoustic delay line-s of the general type mentioned above are preferred, other known types of delay lines may also be employed.

The present pulse decoding circuits have been described as used in a paging system receiver. The circuits are also suitable for use as a party selection component in a mobile radio-telephone receiver. Similarly, they may be advantageously employed in any installation where the weight and power consumption of the required pulse decoding circuits should be held to a minimum.

It is to be understood that the above-described arrange ments are illustrative of the application of the principles of the invention. Numerous other arnangernents may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A radio paging system employing a selective pulse position modulation coding arrangement, said, coding arrangement comprising a plurality of pulse pairs having distinct predetermined time separations between the pulses of each pair and with given time intervals between successive pairs, means for successively receiving said plurality of pulse pairs, an input circuit coupled to said receiving means, a pulse transformer having the primary Winding coupled to said input circuit and having a plurality of secondary windings, timing means connected to said input circuit including a delay line having a delay equal to the time separation between the pulses of the first of the received pulse pairs, gating means connected to one of said secondary windings and to the output of said timing means for providing an energizing signal in response to the presence of concurrent pulse signals at the secondary Winding and the output of said timing means, holding circuit means connected to said gating means and operative in response to said energizing signal to provide an output signal of a duration corresponding to the time interval between the second pulse of the first received pulse pair and the second pulse of the second received pulse pair, a second timing means connected to said input circuit including a delay line having a delay equal to the time separation between the pulses of the second of the received pulse pairs, second gating means connected to another of said secondary windings and to the output circuit of said second timing means for providing an energizing signal in response to the presence of concurrent pulse signals at the secondary winding and the output of said second timing means, second holding circuit means connected to the output of said second gating means and the output of the first-mentioned holding circuit means and operative in response to the presence of concurrent signals at the output of said second gating means and the output of said first-mentioned holding circuit means to provide an output signal of a duration corresponding to the time interval between the second pulse of the second received pulse pair and the second pulse of the third received pulse pair, a third timing means connected to said input circuit including a delay line having a delay equal to the time separation between the pulses of the third of the received pulse pairs, third gating means connected to still another of said secondary windings and to the output of said third timing means for signal in response to the signals at the third timing means, signaling means, and circuit means connected to the output of said third gating means and to the output of said second holding circuit means for delivering an actuating signal to said signaling means in response to the presence of concurrent signals at the output of said third gating means and the output of said sec- 7 0nd holding circuit means.

2. A system as defined in claim 1 wherein each holding circuit means includes a transistor normally biased to cut oil.

References Cited in the file of this patent UNITED STATES PATENTS providing an energizing presence of concurrent pulse secondary winding and the output of said 

1. A RADIO PAGING SYSTEM EMPLOYING A SELECTIVE PULSE POSITION MODULATION CODING ARRANGEMENT, SAID CODING ARRANGEMENT COMPRISING A PLURALITY OF PULSE PAIRS HAVING DISTINCT PREDETERMINED TIME SEPARATIONS BETWEEN THE PULSES OF EACH PAIR AND WITH GIVEN TIME INTERVALS BETWEEN SUCCESSIVE PAIRS, MEANS FOR SUCCESSIVELY RECEIVING SAID PLURALITY OF PULSE PAIRS, AN INPUT CIRCUIT COUPLED TO SAID RECEIVING MEANS, A PULSE TRANSFORMER HAVING THE PRIMARY WINDING COUPLED TO SAID INPUT CIRCUIT AND HAVING A PLURALITY OF SECONDARY WINDINGS, TIMING MEANS CONNECTED TO SAID INPUT CIRCUIT INCLUDING A DELAY LINE HAVING A DELAY EQUAL TO THE TIME SEPARATION BETWEEN THE PULSES OF THE FIRST OF THE RECEIVED PULSE PAIRS, GATING MEANS CONNECTED TO ONE OF SAID SECONDARY WINDINGS AND TO THE OUTPUT OF SAID TIMING MEANS FOR PROVIDING AN ENERGIZING SIGNAL IN RESPONSE TO THE PRESENCE OF CONCURRENT PULSE SIGNALS AT THE SECONDARY WINDING AND THE OUTPUT OF SAID TIMING MEANS, HOLDING CIRCUIT MEANS CONNECTED TO SAID GATING MEANS AND OPERATIVE IN RESPONSE TO SAID ENERGIZING SIGNAL TO PROVIDE AN OUTPUT SIGNAL OF A DURATION CORRESPONDING TO THE TIME INTERVAL BETWEEN THE SECOND PULSE OF THE FIRST RECEIVED PULSE PAIR AND THE SECOND PULSE OF THE SECOND RECEIVED PULSE PAIR, A SECOND TIMING MEANS CONNECTED TO SAID INPUT CIRCUIT INCLUDING A DELAY LINE HAVING A DELAY EQUAL TO THE TIME SEPARATION BETWEEN THE PULSES OF THE SECOND OF THE RECEIVED PULSE PAIRS, SECOND GATING MEANS CONNECTED TO ANOTHER OF SAID SECONDARY WINDINGS AND TO THE OUTPUT CIRCUIT OF SAID SECOND TIMING MEANS FOR PROVIDING AN ENERGIZING SIGNAL IN RESPONSE TO THE PRESENCE OF CONCURRENT PULSE SIGNALS AT THE SECONDARY WINDING AND THE OUTPUT OF SAID SECOND TIMING MEANS, SECOND HOLDING CIRCUIT MEANS CONNECTED TO THE OUTPUT OF SAID SECOND GATING MEANS AND THE OUTPUT OF THE FIRST-MENTIONED HOLDING CIRCUIT MEANS AND OPERATIVE IN RESPONSE TO THE PRESENCE OF CONCURRENT SIGNALS AT THE OUTPUT OF SAID SECOND GATING MEANS AND THE OUTPUT OF SAID FIRST-MENTIONED HOLDING CIRCUIT MEANS TO PROVIDE AN OUTPUT SIGNAL OF A DURATION CORRESPONDING TO THE TIME INTERVAL BETWEEN THE SECOND PULSE OF THE SECOND RECEIVED PULSE PAIR AND THE SECOND PULSE OF THE THIRD RECEIVED PULSE PAIR, A THIRD TIMING MEANS CONNECTED TO SAID INPUT CIRCUIT INCLUDING A DELAY LINE HAVING A DELAY EQUAL TO THE TIME SEPARATION BETWEEN TH PULSES OF THE THIRD OF THE RECEIVED PULSE PAIRS, THIRD GATING MEANS CONNECTED TO STILL ANOTHER OF SAID SECONDARY WINDINGS AND TO THE OUTPUT OF SAID THIRD TIMING MEANS FOR PROVIDING AN ENERGIZING SIGNAL IN RESPONSE TO THE PRESENCE OF CONCURRENT PULSE SIGNALS AT THE SECONDARY WINDING AND THE OUTPUT OF SAID THIRD TIMING MEANS, SIGNALING MEANS, AND CIRCUIT MEANS CONNECTED TO THE OUTPUT OF SAID THIRD GATING MEANS AND TO THE OUTPUT OF SAID SECOND HOLDING CIRCUIT MEANS FOR DELIVERING AN ACTUATING SIGNAL TO SAID SIGNALING MEANS IN RESPONSE TO THE PRESENCE OF CONCURRENT SIGNALS AT THE OUTPUT OF SAID THIRD GATING MEANS AND THE OUTPUT OF SAID SECOND HOLDING CIRCUIT MEANS. 